There are ever increasing methods offered to improve privacy and spectral efficiency in wireless communications. For example, methods such as direct sequence spread spectrum communications, frequency hopped spread spectrum, and burst mode communications are just a few of the waveform solutions for enhanced privacy. Spectral efficiency is achieved by using modulation that is more efficient and coding techniques such as 128-QAM and Turbo codes respectively.
Spread spectrum techniques, which are widely used in signal communications, spread a signal with a relatively narrow bandwidth over a much wider bandwidth, 1000× greater for example. Some applications of spread spectrum includes communications privacy, increasing resistance to noise, interference and jamming of signals. It is also used to prevent signal detection and eavesdropping, and to limit power flux density, for example, in satellite downlink communications. Frequency hopping is one of the modulation techniques used in spread spectrum signal transmission.
In a frequency-hopping spread spectrum (FHSS) communication, radio signals are transmitted by rapidly changing carrier frequencies among many available frequencies, using a pseudorandom sequence known to both the transmitter and the receiver. The overall bandwidth required for frequency hopping is much wider than that required to transmit the same information using only one carrier frequency. However, because transmission occurs only on a small portion of this bandwidth at any given time, the power spectral density is reduced in proportion to the ratio of the hopping bandwidth to the instantaneous bandwidth.
The term mode refers to solutions to the Wave Equation. The Wave Equation is a second order partial differential equation whose solution describes the propagation mode of a wave. The wave equation has an infinite number of solutions, where each solution is a mode. For plane waves, mode zero is the solution. For non-plane waves, there are an infinite number of positive and negative integer solutions.